Automatic bias control

ABSTRACT

Gradual deterioration of performance of an electrophotosensitive member is compensated for in an electrophotographic machine by a device which gradually raises bias on a development electrode according to use of the electrophotosensitive member in the machine. Apparatus is disclosed for counting the number of cycles of operation of the electrophotographic machine and changing the bias applied to magnetic brush in the apparatus in response to a predetermined number of the machine.

0 United States Patent 1 3,575,505

[72] Inventor Charles D. Pannigiani 3,331,355 7/1967 Donalies 355/3X Rochester, N.Y. 3,332,396 7/1967 Gundlach... 355/3X [211 App]. No. 748,796 3,345,925 10/1967 Ostensen 355/10 2: fi d Primary ExaminerSamuel S. Matthews y K k C Assistant Examiner-Kenneth C. Hutchinson 1 sslgnee Emma oda ompmy Attorneys-Robert W. Hampton and Leonard W. Treash, Jr.

Rochester, N.Y.

[54] AUTOMATIC BIAS CONTROL .85 ABSTRACT: Gradual deterioration of performance of an US. Cl. electrophotosensitive member is compensated for in an [5]] G03813/os electrophotographic machine by a device which gradually of 3, raises bias on a development electrode according to use of the 14, 15, q electrophotosensitive member in the machine. Apparatus is disclosed for counting the number of cycles of operation of [56] References CM the electro hoto ra hic machine and changing the bias P g P UNITED STATES PATENTS applied to magnetic brush in the apparatus in response to a 2,956,487 10/1960 Giaimo 355/14 predetermined number of the machine.

EXPOSURE COUNTER PATENTEU APR20 I97! FIG.

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EXPOSURE 600V E 500v 400V 300v 200v CHARLES 0. PARMIGIANI INVENTOR.

BY may ATTORNEYS AUTOMATIC BIAS CONTROL BACKGROUND OF THE INVENTION This invention relates to electrophotography and, more specifically, to a device for controlling thebackground of'an electrophotographically produced image as the image forming quality of a photoconductive member-deteriorates from use.

In electrophotography, it is common to charge a surface of a photoconductive member, expose it to a pattern of radiation forming an electrostatic image, develop the electrostatic image by the application of a toner forming a toner image, and then utilize the toner image, for example, by-transferring it to a receiving surface. It is common to then clean the photoconductive member and reuse it in the same cycle with the same or a new image pattern.

It has been well known for some time that in continued use of the same surface in this process, a thin layer of scum gradually builds up on the surface which scum interfereswith the formation of the electrostatic charge. One of the niost serious problems created with this layer of scum is the reluctance of the areas exposed to light to discharge in the exposing process. The result of this reluctance to discharge is a relatively high potential in the background areas of the electrostatic image and the resulting attraction of toner to these areas. This shows up in the final product as a dark background.

Development electrodes have been applied to development of electrostatic images and can be made to assist in giving a number of advantageous effects. To give clean backgrounds with a fresh photoconductive member, it is common'to bias a development electrode to a point slightly above the minimum potential of the exposed electrostatic image. This bias discourages toner particles from being attracted to the background portions of the image. However, as the bias is raised further, it will tend to inhibit some of the toner from being attracted to the image areas, thereby decreasing density in image areas. This phenomenon is not as noticeable in a final transferred image as a dark background. Bias is" chosen to balance these two effects.

SUMMARY OF THE INVENTION It is an object of this invention to provide an electrophotographic machin'ein which the effect of scumming on final image quality is sharply reduced over a much longer life for a given photoconductive member.

This and other objects are accomplished by automatically compensating for expected scumming of the photoconductive member by raising the bias on a development electrode according to the use of the photoconductive member.

In a preferred embodiment, the bias control is automatically raised according to a counter linked to the number of copies produced by the machine.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a schematic diagram of an electrophotographic machine illustrating its major components.

FIG. 2 is a graph showing discharge of a photoconductive member according toe'xposure.

FIG. 3 is a graph illustrating the effect of repetitive use of the photoconductive member illustrated in FIG. 2.

FIG. 4 is a schematic diagram of a development station in an electrophotographic machine illustrating the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. I, a photoconductive member 1 is made up of a photoconductive layer 2 and a' conductive backing 3 coated on a suitable flexible support, not illustrated in FIG. 1. Examples of photoconductors with which the invention is useful and methods for manufacturing and coating them are described in US. Pat. No. 3,141,770, Davis, and French Pat. No. 1,383,461. Examples of conductive layers and methods of coating them on a suitable support are described in U.S. Pat.

No. 3,245,833, Trevoy. The photoconductive member 1 is mounted on spaced rollers 4 and 5 to be driven through an endless path by a motor 20 into cyclical operative relation with a series of stations. At a corona charging station 6, the photoconductive member 1 is given a uniform charge. It then passes to an exposure station 7'where it is imagewise exposed to actinic radiation, thereby forming an electrostatic image by discharge of the areas exposed. The exposed photoconductive meinber 1 then passes to a development station 8, which can include, for example, two magnetic brushes 9 and 10, where the image is developed by application of finely divided toner particles. The resulting toner image then passes to a transfer station 11 where it is transferred to a receiving surface on a receiving sheet l2and is then fused to the receiving surface at a fusing station 19. The photoconductive member 1 is'th'en cleaned of residual toner by a brush cleaner 13 and is ready for reuse in the process. Similar processes are known in which certain of these stations are eliminated, for example, the cleaning, transfer and/or fusing stations.

As the apparatus shown in FIG. 1 is repetitively used in the process described, it has been found that a layer of scum gradually builds up on the surface to be charged. This layer of scum is believed to be at least partially due to a small amount of fusion of toner to the photoconductive member when continuously rubbed by the magnetic brushes 9 and 10. However, it is also found to be present in varying degrees in machines using other forms of development, for example, cascade development. The effect of this scum is illustrated by the graphs shown in FIGS. 2 and 3. Referring to FIG. 2, a fresh photoconductive member is given an initial charge of 600 volts of either polarity. As any particular area is exposed, the charge is reduced'on that area according to a curve A. The curve has a substantial toe at the volt level, indicating that discharge beyond the level of 100 volts does not occur in an ordinary exposure. If the charged member is exposed to a pattern from an original having continuously varying density, an electrostatic image is formed which varies from the areas of greater density, the image areas, to the areas of lesser density, the background areas, according to a curve shown as curve B in FIG. 2with the areas receiving the least amount of radiation having a potential of approximately 400 volts and those receiving the most amount of light having a potential of about I00 volts.

Referring now to FIG. 3, the effect of scumming on discharge is illustrated. As scum builds up on the photoconductor, the ability of the photoconductor to accept an original charge is increased slightly to 650 volts, but the background charge remaining after complete exposure has also increased to I50 volts. Discharge during exposure of a scummed photoconductor is shown by curve C. Actual exposure to an image of continuously varying density will generally result in about the same charge in the image area as with a fresh photoconductor, 400 volts, but the background charge or toe has been raised to 150 volts giving a curve D substantially different in its toe from curve B.

When a bias is placed on a development electrode in the developing station slightly more than the expected background potential, say volts for a fresh photoconductor, toner particles are attracted away from the background areas to the development electrode leaving the background clear. In the case of FIG. 1 apparatus, the magnetic brush itself serves as the development electrode. However, when scumming raises the voltage in the background areas of the electrostatic image to a point equal to or above the bias, toner particles are no longer attractedaway from the background areas and are deposited in the background areas of the toner image, giving a print with an unclean background, one of the most serious defects in a reproduction. v

When this happens in electrophotographic equipment, it is presently necessary to either replace the photoconductive member or clean off the scum witha solvent. According 'to the' invention, the amount of use obtainable from the photoconductive member can be greatly increased by gradually raising the bias of the development electrode over thelife of a photoconductive member according to the expected scumming in any particular application. This allows very low bias control with a fresh photoconductive member with the advantages of dark image areas, while automatically providing a sacrifice of this feature for a clean background as the photoconductive member is used and scum builds up.

Referring now to FIGS. 1 and 4, a device is provided which automatically raises the bias on a development electrode according to the amount of use of a photoconductive member. As described above, the toning station 8 can contain two magnetic brushes 9 and 10, each of which are adapted to use a developer mix of ferromagnetic carrier particles and finely divided toner particles. The toner and carrier are chosen to triboelectrically impart a desired charge to the toner. In operation, the toner-carrier mix is transported across the electrostatic image carried on the photoconductive member 1. Referring to FIG. 4, the electrostatic image is shown as a large concentration of negative charge 14 in the image areas and a small amount of negative charge 15 in the background areas. As is well known in the art, a bias is applied to the magnetic brushes 9 and from any suitable voltage source, for example, a battery 28 connected between a conductive backing 3 and each magnetic brush. As is also well known in the art, the same effect is obtained if the conductive backing is grounded and the bias applied to the magnetic brush with respect to ground as is illustrated in FIG. 1. The bias is made variable by known means, for example, by taking the bias from a variable resistance 21 connected in series with the battery 16. In each case, a bias is applied of the same polarity as the charge on the photoconductive member 1 and of slightly higher potential than that of the background areas with a fresh photoconductor. As the machine is continually used in the process, the amount of use of the photoconductor is logged by suitable means, for example, a counter 17 linked to a switch 18 actuated as each copy comes out of the fusing station 19. After a given number of copies, for example, 1,000, a signal is produced by known means in the counter 17 which is used to adjust the voltage applied between the magnetic brushes and the conductive layer 3. The voltage is raised a small amount to be comparable to the rise in background voltage expected because of scumming after 1,000 copies have been made. Referring to FIG. 1, this can be accomplished by a motor 22 which slightly lowers the resistance 21, thereby raising the potential applied to the brushes 9 and 10. Alternatively, referring to FIG. 4, the signal from the counter 17 can be used to actuate a solenoid 24 which rotates a ratchet 25 against a spring 26 to change a tap 27 on a variable voltage source 28. When the expected life of the photoconductive member has been used, the same counter can be used to provide a signal to a control panel of the machine indicating that the photoconductive member should be replaced.

It can be seen that the adjustment in bias can be linked to operation of any of a number of elements of the machine. For

example, it can be linked to the drive train of the machine itself by counting the number of revolutions of one of the rollers 4. Since the developing mechanism is working all the time the photoconductive member is running, the same result will be obtained if the bias control is linked to revolutions of the magnetic brushes 9 and 10. A somewhat different result is obtained if the bias control is linked to a toner dispensing mechanism which adds toner as needed by the developing station. With such an arrangement, the bias would then be raised according to the amount of toner used, which will vary according to the type of reproductions made. These examples are intended to be illustrative only of a large number of elements to which the bias control can be linked for monitoring of the use of the photoconducting member.

The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention as described hereinabove and as defined in the appended claims.

Iclaim:

1. In electrophotographic apparatus for use with a photoconductive member having a surface adapted to hold a charge, said apparatus having means for repetitively cycling said member through operative relation with a series of electrophotographic components, said series of components including:

means for applying a charge to said surface,

means for exposing said member to a pattern of actinic radiation to create an electrostatic image on said surface in conformity with said pattern,

means for developing said electrostatic image forming a powder image defined by said electrostatic image, said developing means including:

a. a development electrode,

b. means for applying an electrical bias to said development electrode, and

means for utilizing said powder image, the improvement including:

means for raising incrementally the bias on said development electrode, and

means for sensing operating cycles of said apparatus and for actuating said raising means periodically according to a predetermined number of cycles of said apparatus.

2. The improvement according to claim 1 wherein said apparatus is a copying machine and said sensing means includes means for counting the number of copies made by said apparatus to raise said bias each time a predetermined number of copies is made.

3. In an electrophotographic machine, in which electrostatic images are created by imagewise exposing a charged photoconductive surface disposed on a conductive backing and said electrostatic images are developed by the application of toner by a developing means which developing means includes a development electrode and a means for applying a bias to said development electrode, the improvement comprising:

means coupled to said bias applying means actuatable in response to a predetermined number of operations of an element of said machine to raise the bias applied to said development electrode.

4. An electrophotographic apparatus including:

a photoconductive member having a charge-holding surface,

means for applying a charge to said surface,

means for exposing said surface to a pattern of actinic radiation to create an electrostatic image on said surface having a minimum potential,

means for developing said electrostatic image forming a powder image, said means including:

a. a development electrode,

b. means for applying an electrical bias to said development electrode,

means for transferring said powder image to a receiving surface,

means for cleaning said charge-holding surface of residual powder so that said surface may be reused,

means for repetitively cycling said member through said apparatus, wherein said charge-holding surface gradually builds up a scum which causes said minimum potential to gradually increase, and

means coupled to said bias applying means actuatable in response to a predetermined number of operatioirs of an element in said machine to raise incrementally the bias on said development electrode to compensate for the increase in said minimum potential.

5. The apparatus according to claim 4 wherein said means for raising the bias is actuated by a predetermined number of receiving surfaces passing through the transfer means.

6. In an electrophotographic apparatus for making copies from one or more originals, said apparatus being adapted to receive a sensitive member having at least a conductive layer and a photoconductive layer, said apparatus having means for repetitively cycling said member through operative relation with a series of electrophotographic stations which series includes:

means for applying a charge of a first polarity to said sensitive member,

means for exposing said member to a radiation image from surface to form thereon a copy of said original, the

improvement including:

means for counting the number of copies made with a specific sensitive member, and

means coupled to said counting means to raise the bias on said development electrode according to the number of copies made on said sensitive member.

7. ln electrophotographic apparatus of the type operable in repetitive cycles in which electrostatic images are created by imagewise exposure of a charged photoconductive surface disposed on a conductive backing and developed by a developing mechanism of the type having magnetic brush means for contacting the electrostatic image with toner and means for applying an electrical bias to said magnetic brush means, the improvement comprising:

a. means for sensing the number of apparatus cycles following a reference cycle; and

b. means, cooperating with said bias applying means and responsive to a sensing of a predetermined number of apparatus cycles by said sensing means, for raising the electrical bias applied to said magnetic brush means. 

1. In electrophotographic apparatus for use with a photoconductive member having a surface adapted to hold a charge, said apparatus having means for repetitively cycling said member through operative relation with a series of electrophotographic components, said series of components including: means for applying a charge to said surface, means for exposing said member to a pattern of actinic radiation to create an electrostatic image on said surface in conformity with said pattern, means for developing said electrostatic image forming a powder image defined by said electrostatic image, said developing means including: a. a development electrode, b. means for applying an electrical bias to said development electrode, and means for utilizing said powder image, the improvement including: means for raising incrementally the bias on said development electrode, and means for sensing operating cycles of said apparatus and for actuating said raising means periodically according to a predetermined number of cycles of said apparatus.
 2. The improvement according to claim 1 wherein said apparatus is a copying machine and said sensing means includes means for counting the number of copies made by said apparatus to raise said bias each time a predetermined number of copies is made.
 3. In an electrophotographic machine, in which electrostatic images are created by imagewise exposing a charged photoconductive surface disposed on a conductive backing and said electrostatic images are developed by the application of toner by a developing means which developing means includes a development electrode and a means for applying a bias to said development electrode, the improvement comprising: means coupled to said bias applying means actuatable in response to a predetermined number of operations of an element of said machine to raise the bias applied to said development electrode.
 4. An electrophotographic apparatus including: a photoconductive member having a charge-holding surface, means for applying a charge to said surface, means for exposing said surface to a pattern of actinic radiation to create an electrostatic image on said surface having a minimum potential, means for developing said electrostatic image forming a powder image, said means including: a. a development electrode, b. means for applying an electrical bias to said development electrode, means for transferring said powder image to a receiviNg surface, means for cleaning said charge-holding surface of residual powder so that said surface may be reused, means for repetitively cycling said member through said apparatus, wherein said charge-holding surface gradually builds up a scum which causes said minimum potential to gradually increase, and means coupled to said bias applying means actuatable in response to a predetermined number of operations of an element in said machine to raise incrementally the bias on said development electrode to compensate for the increase in said minimum potential.
 5. The apparatus according to claim 4 wherein said means for raising the bias is actuated by a predetermined number of receiving surfaces passing through the transfer means.
 6. In an electrophotographic apparatus for making copies from one or more originals, said apparatus being adapted to receive a sensitive member having at least a conductive layer and a photoconductive layer, said apparatus having means for repetitively cycling said member through operative relation with a series of electrophotographic stations which series includes: means for applying a charge of a first polarity to said sensitive member, means for exposing said member to a radiation image from an original to be copied to create an electrostatic image on said member, means for developing said electrostatic image forming a powder image defined by said electrostatic image, said developing means including a development electrode and means for applying an electrical bias of said first polarity to said development electrode, means for transferring said powder image to a receiving surface to form thereon a copy of said original, the improvement including: means for counting the number of copies made with a specific sensitive member, and means coupled to said counting means to raise the bias on said development electrode according to the number of copies made on said sensitive member.
 7. In electrophotographic apparatus of the type operable in repetitive cycles in which electrostatic images are created by imagewise exposure of a charged photoconductive surface disposed on a conductive backing and developed by a developing mechanism of the type having magnetic brush means for contacting the electrostatic image with toner and means for applying an electrical bias to said magnetic brush means, the improvement comprising: a. means for sensing the number of apparatus cycles following a reference cycle; and b. means, cooperating with said bias applying means and responsive to a sensing of a predetermined number of apparatus cycles by said sensing means, for raising the electrical bias applied to said magnetic brush means. 